Letters of Intent for 2015

LoI 2015-169
Focus meeting: what is a galaxy?


3 August 2015 to 14 August 2015


Honolulu (as part of GA), United States


Pierre-Alain Duc (paduc@cea.fr)

Coordinating division:

Division J Galaxies and Cosmology

Co-Chairs of SOC:

Pierre-Alain Duc (AIM Paris-Saclay)
Duncan Forbes (Swinburne University)

Chair of LOC:




Properties of low mass "galaxies" and high mass star "clusters"
(Ultra Faint Dwarf galaxies, Ultra Compact Dwarf Galaxies, Tidal Dwarf Galaxies, low surface brightness galaxies, intergalactic HI structures, intergalactic Globular Clusters):
stellar content, scaling relations, internal kinematics and dark matter content.

Discussion on the definition of galaxies



In the recent years, new types of stellar objects were identified that make the borderline between star clusters and galaxies very elusive. For example: the Ultra Faint Dwarf Galaxies in the Local Group have stellar masses lower than that of the typical Milky Way Globular Clusters (GCs), but have the highest measured Mass to Light (M/L) ratios. Cases of intergalactic Globular Clusters have been found. The Ultra Compact Dwarf Galaxies (UCDGs) have sizes and M/L ratios closer to that of GCs than dark-matter dominated galaxies. The Tidal Dwarf Galaxies (TDGs) have the total mass, gas and stellar content of regular dwarf galaxies, but do not contain dark matter.The isolated gas-rich objects, such as the High Velocity Clouds (HVCs), low Surface Brightness Galaxies (LSBs), or the extreme controversial "Dark Galaxies", may qualify or not as galaxies, depending on their debated dark matter content. On the other hand cosmological models predict the existence of many galaxy satellites with a very small baryon fraction, which have presumably not yet been found.

Several recent papers have thus proposed to revise the definition of galaxies to take into account these peculiarities and argued on criteria based on dynamical or stellar population criteria, rather than on their dark matter content. However, no consensus has yet been reached. The IAU General Assembly that gathers scientists from different fields would in fact be the ideal place to host a discussion on the definition of galaxies. The strong debate on the nature of Pluto during the IAU GA in Prague indicates that "words matter".
Our proposed focus meeting may attract specialists of galaxies, dwarfs and star clusters that would have there the possibility to present their latest results on their key transition objects, but also simulators interested in the formation process of galaxies/star clusters as well as CDM supporters and opponents.

A 1-2 day meeting with 3 sessions would be ideal, with a session on star and globular clusters, one on galaxies (Ultra faint dwarfs, ultra compact dwarfs, tidal dwarf galaxies), while the last session would address the very definition of galaxies

IAU General Assembly
Honolulu, 3-14 August 2015

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